Electric arc furnaces have long been used, for example for steel manufacture. Such a furnace has one or more electrodes and a furnace container, in which the charge of the furnace is applied. The electrode or electrodes is or are connected to an electric power network, which feeds current to one or more arcs which are burning between the electrodes and the charge and/or between the electrodes. The electrical energy developed in the arcs results in the desired heating of the furnace charge.
Arc furnaces are usually supplied with alternating voltage, but also arc furnaces supplied with direct voltage occur. A typical arc furnace for use during steel manufacture has three electrodes arranged above the charge, which are each connected to their own phase in a three-phase power-supply network. The electrodes can be provided in a known manner with devices for position control of the electrodes, that is, for control of their distances to the charge and hence of the length of the arcs. The furnace is usually connected to the network via a furnace transformer, which is normally provided with an on-load tap changer for control of the voltage supplied to the furnace.
Arc furnaces are often designed for very high power, and the effect of such a furnace on the power-supply network, as well as its effect on other consumers connected to this network, may therefore be strong.
An arc furnace exhibits, especially during certain types of operation such as, for example, during melting of a charge, unstable operating conditions. Short circuits between the electrodes and the charge frequently occur, with surge currents resulting therefrom. Also, it may occur that an arc is extinguished, whereby the current in the phase in question ceases altogether. Usually, these disturbances are unsymmetrical in such a way that two phases become short-circuited and one phase currentless. These phenomena give rise to considerable load variations in the power-supply network, and the load variations cause voltage variations in the network.
Low-frequency components of the above voltage variations can be reduced with the aid of equipment for controllable static reactive-power compensation. However, a considerable part of the voltage variations caused by an arc furnace lie within a frequency range--about 4-8 Hz--where the voltage variations cause so-called flicker, which is disturbing to the eye, that is, variations in light intensity of light bulbs and other light sources, as well as other disturbances, for example changes in the appearance of the picture in TV receivers, while at the same time the frequency of the disturbances is so high that it is not possible to reduce the disturbances to the desirable extent with the aid of controllable reactive-power compensation.
Unless the short-circuit power of the electric power-supply network is very large in relation to the rated power of the furnace, an arc furnace will, for the above reasons, cause inconvenient disturbances for the other subscribers.
After the meltdown phase, the arcs of the furnace burn in a more stable manner, which reduces the above-mentioned disturbances. During this phase, an arc is extinguished at each zero crossing of the electrode current and is regularly restruck after a certain time during the next half-cycle of the electrode voltage, when the voltage with the new polarity has reached a certain value sufficient for firing. A currentless interval appears between each extinction and the following firing, that is, once every half-cycle. These currentless intervals reduce the average power developed in the arcs and hence the production capacity of the furnace. It is therefore desirable to reduce the length of the currentless intervals, if possible.
It is previously known that, by connecting additional inductances into the supply lines of the electrodes, faster restrikings after the zero crossings of the current can be obtained. This results in a reduction of the length of the currentless intervals. However, for the furnace equipment as a whole, the method entails a lower power factor and a lower current amplitude but, on the other hand, it results in longer arcing times and hence a possibility of increased furnace production. At the same time, the costs of the furnace equipment, and the losses therein, increase.